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Codd’s 12 Rules for Relational Databases

In October 1985, Codd presented 12 rules that a database
must obey if it is to be considered truly relational:

Information
rule—All
information in a relational database is represented explicitly at the logical
level and in exactly one way — by values in tables.

Guaranteed
access rule—Each and every datum (atomic value) in a relational database is guaranteed to
be logically accessible by resorting to a combination of table name, primary
key value, and column name.

Systematic
treatment of NULL values—NULL values are supported in a fully relational DBMS for representing
missing information and inapplicable information in a systematic way,
independent of the data type.

Dynamic online catalog based on the relational model—The database description is
represented at the logical level in the same way as ordinary data, so that
authorized users can apply the same relational language to its interrogation as
they apply to the regular data.

Comprehensive data sublanguage rule—A relational system may
support several languages and various modes of terminal use (for example, the
fill-in-the-blanks mode). However, there must be at least one language whose
statements are expressible, per some well-defined syntax, as character strings,
and that is comprehensive in supporting all of the following items:

Data definition

View definition

Data manipulation (interactive and
by program)

Integrity constraints

Authorization

Transaction boundaries (begin, commit, and rollback)

View
updating rule—All views
that are theoretically updateable are also updateable by the system.

High-level
insert, update, and delete—The capability of handling a base relation or
a derived relation as a single operand applies not only to the retrieval of
data, but also to the insertion, update, and deletion of data.

Physical
data independence—Application programs and terminal activities remain logically unimpaired
whenever any changes are made in either storage representations or access
methods.

Logical
data independence—Application programs and terminal activities remain logically unimpaired
when information-preserving changes of any kind that theoretically permit
unimpairment are made to the base tables.

Integrity
independence—Integrity constraints specific to a particular relational database must be
definable in the relational data sublanguage and storable in the catalog, not
in the application programs.

Nonsubversion
rule—If a relational system has a low-level (single record at a time) language,
that low level cannot be used to subvert or bypass the integrity rules and
constraints expressed in the higher-level relational language (multiple records
at a time).